Reinforced optical fiber cable with glass or silica core

ABSTRACT

A reinforced optical fiber cable comprises (i) an optical filamentary material with a glass or silica core and a lower index of refraction sheath, (ii) a reinforcement comprising at least two polymeric fibers having an elastic modulus above 10,000,000 psi, said fibers being held under tension .[.separate from one another.]. .Iadd.around the sheath .Iaddend.and positioned substantially parallel to the longitudinal axis of the core with substantially zero twist and (iii) a jacket which holds the reinforcement under tension.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of my prior copending United Statesapplication Ser. No. 734,749 filed Oct. 22, 1976, now abandoned.

BACKGROUND OF THE DISCLOSURE

The present invention relates to an optical fiber cable containing atleast one optical filamentary material having a glass or silica core anda lower index of refraction sheath.

Optical filamentary materials are well known in the art for transmissionof light along a filament length by multiple internal reflections oflight. Great care is taken to minimize light losses along the length ofthe filament or, in other words, internal reflections are made as totalas possible so that light applied to one end of the optical filamentarymaterial is efficiently transmitted to the opposite end of the material.The light transmitting portion or core of the optical filamentarymaterial is surrounded by a sheath having a lower index of refractionwhich minimizes the escape or absorption of light along the length ofthe filament. This sheath is normally transparent since an opaque sheathtends to absorb light. Although this sheath can be made from glass or apolymeric material, it is conventionally made from the latter due toincreased toughness properties.

Optical filamentary materials can be divided into two general classesdependent upon the type of optically transparent core material. A firstclass of core material is thermoplastic in nature while a second classis made from glass or silica. The first class is generally superior bothin toughness and in ease of making connections while the second class isgenerally superior in light transmission.

One disadvantage with optical filamentary materials with a glass orsilica core is a tendency for the core to break due to its brittleness.Encapsulation of the filaments within a cable containing reinforcementand a protective layer has only been partially successful in overcomingthe brittle quality of the core. A need exists for an optical fibercable which provides increased resistance to breakage of a brittle corematerial.

SUMMARY OF THE INVENTION

The present invention relates to a cable for transmission of lightcomprising

(A) a substantially cylindrical core of an optically transparent glassor silica,

(B) a transparent sheath for (A) having an index for refraction at least0.1% lower, and

(C) a reinforcement for the cable,

(D) a jacket exterior of (A) and (B),

wherein the improvement comprises the reinforcement of (C) comprising atleast two polymeric fibers spaced from one another

(i) having an elastic modulus of at least 10,000,000 psi;

(ii) being under tension;

(iii) being substantially parallel to the core along its longitudinalaxis;

(iv) being positioned with substantially zero twist;

(v) being interspaced between (B) and (D).

DETAILED DESCRIPTION OF THE INVENTION

An optically transparent cylindrical core for transmission of light ismade from an optically transparent glass or silica. The silica core canbe either pure silica (undoped) or doped with a suitable component suchas germanium or boron. As employed herein "optically transparent" meansa light transmission of at least 50% per 30 cms in a portion of thelight spectrum of 550 to 1100 nanometers. This degree of transmissionneed not extend over the entire spectrum. Examples of suitabledisclosure of core materials are found in U.S. Pat. Nos. 3,480,458 and3,508,589, e.g., the latter patent lists suitable core materials madefrom barium, flint and borosilicate glasses with the more dense glassesdescribed as better.

A preferred core material is made from silica which can be either dopedor undoped. The silica is drawn into a core material at elevatedtemperature. Although drawing temperatures of at least 2000° C. can beemployed, a temperature range of 2040° C. to 2120° C. is preferred. Asdrawing temperature decreases, it has been found that brittleness of thedrawn silica core material increases. A limiting factor on an uppertemperature range is difficulty in control of caliper. As the drawingtemperature is maximized, a necessary degree of caliper control becomesmarginal.

The diameter of the cylindrical optically transparent core varies fromrelatively thin to relatively thick core constructions. A suitablediameter range is 10 to 400 μm. A thick core has the advantage in theability to capture a greater proportion of incident light if the lightsource is large, e.g., from an LED (light emitting diode) but has thedisadvantage of having a larger bending radius. If a light source issmall, e.g., a laser, a relatively thin core is suitable for capturingincident light.

The sheath applied to the optically transparent core is transparent andhas an index of refraction at least 0.1% lower and can be glass, silicaor a substantially amorphous optically transparent thermoplasticpolymeric material. Pure silica has a lower index of refraction thanmost known glasses, and if silica is employed for both core and sheath,the silica core is doped to raise its index of refraction to a requiredlevel at least 0.1% above the sheath.

Preferred as a material of construction for the sheath is asubstantially amorphous transparent thermoplastic polymer since suchpolymer does not possess the brittleness characteristic of glass orsilica.

Examples of suitable sheath materials include those disclosed in BritishPatent Specification No. 1,037,498 such as polymers and interpolymers ofvinyl fluoride, vinylidene fluoride, tetrafluoroethylene,hexafluoropropylene, trifluoromethyltrifluorovinyl ether,perfluoropropyltrifluorovinyl ether and fluorinated esters of acrylic ormethacrylic acids having the structure ##STR1## wherein X is selectedfrom the group consisting of F, H, or Cl, and n is an integer of from 2to 10, m is an integer from 1 to 6 and Y is either CH₃ or H.

Since the sheath material reflects light traveling through the core, thethickness of the sheath is not generally critical. An example of asuitable range of thickness of this sheathing is 2 to 500 microns.Excessive sheathing thicknesses can reduce flexibility of the finalcable which is undesirable.

Well-known techniques for application of the sheath material aresuitable. Glass or silica can be applied by a double crucible drawingwhile a polymer can be extruded onto the core.

In the present invention, it is necessary to incorporate a reinforcementbetween the optical filamentary material and the protective jacket. Thisreinforcement comprises polymeric fiber having an elastic modulus of atleast 10,000,000 psi. Suitable polymer for the fibers which meet thiscriteria include poly(p-phenylene terephthalamide) and is disclosed inU.S. Pat. No. 3,869,430. The disclosure of this patent is incorporatedby reference herein.

At least two separate individual fibers are employed and are held undertension in the cable by the jacket material. .[.The.]. .Iadd.When twofibers are employed, .Iaddend.fibers are spaced apart and do not contactone another. Preferably, separate bundles of fibers, i.e., yarns, areemployed as opposed to individual fibers. Although two separate fibersor yarns are suitable, more preferably at least four separate fibers oryarns are employed and most preferably, six or more fibers or yarns.These fibers are positioned substantially parallel to the core along itslongitudinal axis. The fibers in relationship to the longitudinal axisof the core will have substantially zero twist. The term "zero twist"means that a fiber would not encircle the core material irrespective ofthe length of such material.

The purpose of having the fibers substantially parallel to thelongitudinal axis of the core with substantially zero twist is to ensurethe fibers remain under tension in the optical fiber cable. If thefibers for reinforcement encircled the core material, relaxation of thisreinforcement could readily occur. Although the degree of tension of thereinforcement fiber is not critical, nevertheless it is essential thatthe fibers remain under tension in the cable. This tension can readilybe demonstrated in a final cable. The cable is cut crosswise and theoptical filamentary material of the glass or silica core and its sheathcan be physically felt to protrude slightly from the cut end of thecable.

The optical filamentary material of a glass or silica core and lowerindex of refraction sheath is positioned within a protective jacket. Thejacket serves to hold the reinforcement under tension and with thisproviso the material for the jacket is not critical. The jacket isconventionally a thermoplastic polymer applied by extrusion. Suitablematerials of construction include polyamides, copolyetheresters,polyurethanes, polyolefins (homopolymers and copolymers includingionomers) such as polyethylene and polypropylene, and melt extrudablefluorocarbons such as tetrafluoroethylene/hexafluoropropylene copolymersand melt extrudable chlorine-containing polymers such as polyvinylchloride.

Considerations which govern the choice of jacketing material includestrength, elongation, burning rate and ease of strippability. Forexample, good strippability is needed in connecting one cable to anotherand in connecting a cable to a light source or detector.

The optical fiber cable of the present invention provides a cable with aglass or silica core which is highly resistant to breakage of this lighttransmission portion. Cables containing an optical filamentary materialof a glass or silica core and a lower index of refraction sheath areknown in the prior art. In the cable of the present invention the mannerof reinforcement in protection of a glass or silica core results in asuperior ability to withstand breakage of the core material incomparison to prior art cables used for the transmission of light withan identical core and sheath material.

The cable of the present invention has a combination of high bendingstrength, high tensile strength and high impact strength. Thiscombination of properties has not been obtained by cables with a brittlecore which do not have reinforcement fibers held under tension.

It is possible with the construction of the present optical fiber cableto obtain a construction which permits the cable to be bent sharplywithout damage. A minimum bend diameter at least equal to about 6 mm andpreferably at least equal to about 4 mm can be obtained. As shown inExample 2, the cable can be tied into a tight overhand knot, e.g. havinga minimum bend diameter at least equal to about 4 mm without the cablelosing its ability to function in a normal fashion in transmission oflight.

Although the disclosure herein has been directed to interspacing areinforcement under tension between a sheath of an optical filamentarymaterial and a jacket, it is understood that the reinforcement need notcontact the sheath. A protective layer can separate the sheath from thereinforcement. In such case, it remains critical for the reinforcementfibers to be maintained under tension.

Also, it is within the scope of the present invention to employ morethan one optical filamentary material within a cable provided suchfilamentary material has a separate reinforcement of at least tworeinforcement fibers in the manner disclosed herein.

To further illustrate the present invention, the following examples areprovided.

EXAMPLE 1

Part I-An undoped silica fiber was spun at a temperature of 2050° C.from 9 mm. rod using a furnace with a tungsten heating element blanketedwith nitrogen. The rod feed to the furnace and the take-off for thefiber were set to make 200 μm fiber at about 10 meters per minute.During spinning less than one break per 1000 meters was observed. Thefiber was solution coated with a lower index substantially amorphoustransparent polymeric sheath of methyl methacrylate and fluorinatedesters of methacrylic acid (second order transition temperature of 50°C. and refractive index 6% lower than core) in a solvent ofdifluorotetrachloroethane to make an optical fiber with about a 600 μmouter diameter.

The optical fiber had an attenuation of 38 dB per kilometer at 655.3 nm.

Part II-The optical fiber of Part I was reinforced with six strands ofpoly(p-phenylene terephthalamide) of 42 tex (380 denier) and jacketedwith copolyetherester (disclosed in Example 1 of U.S. Pat. No.3,651,014).

Six strands of poly(p-phenylene terephthalamide) were initially strungthrough tensioning holders, through a fiber guide which was a hypodermicneedle 1550 μm I.D. (inner diameter), 2050 μm O.D. (outer diameter) andthrough a crosshead die with an 1875 μm hole. Tension of the yarns wasset at 1.16×10⁻³ Newtons/tex (0.013 gram/denier) and thecopolyetherester which was heated to 205° C. was extruded from theopening of the die. The extrusion speed and the speed of the yarns wereadjusted to give an extrudate 175 μm O.D. A blank nylon filament 550 μmO.D. was fed into the yarn bundle and the speed readjusted to give anextrudate 1875 μm in diameter. The die was adjusted to center the fiberand the yarns. The nylon filament was removed and a coated optical fiberof Part I was substituted and coated with the copolyetherester to forman optic fiber cable.

The optic fiber cable had an attenuation of 40 dB per kilometer at 655.3nm (in comparison to 38 dB per kilometer of the Part I optic fiber). Thecable was tested under a load and it broke at 30 kg. The cable could behammered without destroying its ability to transmit light. The cablecould be wrapped around a 6 mm diameter mandrel without breakage of thecore and loss of ability to transmit light but the cable could not betied into a tight knot without breaking the core.

EXAMPLE 2

The procedure of Example 1--Parts I and II were followed except thatExample 1--Part I optical fiber was directly coated withcopolyetherester (described in Example 1 of U.S. Pat. No. 3,651,014) bya tubing cross-head die prior to the method of Example 1--Part II inapplication of the reinforcement and the jacketing copolyetherester. Theoptical fiber had an O.D. of 1225 μm. In the Example 1--Part II methodof reinforcement, yarns of poly(p-phenylene terephthalamide) fibers wereused, three yarns of 42 tex (380 denier) and three yarns of 168 tex(1420 denier). The tension of the fibers was 1.8×10⁻³ newtons/tex (0.02g/denier).

The final optic fiber cable had an O.D. of 2375 μm, an attenuation of 40dB per kilometer at 655.3 nm and a break strength of 85 kg. The cablecould be wrapped on a 4 mm diameter mandrel and tied into a tightoverhand knot without breaking or loss of light transmission.

What is claimed is:
 1. A cable for transmission of light comprising:(A)a substantially cylindrical core of an optically transparent glass orsilica; (B) a transparent sheath for (A) having an index of refractionat least 0.1% lower; (C) a protective layer on (B); (D) a reinforcementfor the cable, on (C), of at least two polymeric fibers spaced .[.fromone another.]. .Iadd.around the sheath, .Iaddend.and(i) having anelastic modulus of at least 10,000,000 psi, (ii) being under tension,.Iadd.such that if the cable is cut crosswise the core and the sheathprotrude slightly from the cut end of the cable, .Iaddend.(iii) beingsubstantially parallel to the core along its longitudinal axis and, (iv)being positioned with substantially zero twist; and (E) a jacketexterior of (A), (B), (C) and (D), holding (D) under tension.
 2. Thecable of claim 1 wherein said fibers are poly(p-phenyleneterephthalamide).
 3. The cable of claim 1 wherein said fibers arepresent in separate yarns.
 4. The cable of claim 3 wherein at least fouryarns are present.
 5. The cable of claim 4 wherein at least six yarnsare present.
 6. The cable of claim 1 with a minimum bend diameter atleast equal to about 6 mm.
 7. The cable of claim 6 with a minimum benddiameter at least equal to about 4 mm.
 8. The cable of claim 1 whereinsaid transparent sheath is a substantially amorphous thermoplasticpolymer extruded onto said core.
 9. The cable of claim 1 wherein saidprotective layer is a copolyetherester.
 10. A fiber optical cablecomprising:a fiber optic core including at least one optical fiberadapted to receive optical communication signals; at least two elasticstrength members substantially straight and parallel to said fiber opticcore; said strength members being in a stretched condition; a protectivelayer between the core and the elastic strength members; means formaintaining said strength members substantially straight and in astretched condition whereby said strength members are adapted to apply aforce in the opposite direction to longitudinal stress forces on saidcable for protecting said optical fiber from longitudinal stress.
 11. Acable as set forth in claim 10 wherein said at least two elasticstrength members is taken from the group consisting of Kevlar®, nylonand polyester.
 12. A cable as set forth in claim 10 wherein said atleast two elastic strength members has a tensil modulus in the range of7×10⁶ pounds per square inch to 19×10⁶ pounds per square inch.
 13. Acable as set forth in claim 10 wherein said at least two elasticstrength members will not substantially elongate under forces equal tothe break strength of the fiber optic core.
 14. A fiber optic cablecomprising:a fiber optic core having at least one glass fiber; aplurality of strength members, said strength members being substantiallystraight and having been stretched beyond their normal length; aprotective layer between the core and the strength members; means formaintaining said strength members substantially straight and in astretched condition; and a protective sheath surrounding said core andsaid strength members.